paper-lexicon
Here you can find some explanations of the most important technical terms in the paper industry.
| A-L | M-S | T-Z |
| grammage | surfaces of paper | volume |
| grain direction | opacity | WFC (wood free coated) |
| WFU (wood free uncoated) |
grain direction
Grain direction is the alignment of fibers in a sheet or web of paper, generated during paper making process.
When paper is cut into sheets, it will be either
- long-grain (if the fibres are aligned parallel to the sheet's longer dimension) or
- short-grain (if the fibres are aligned parallel to the sheet's shorter dimension).
Grain direction is important for the application - for example for books usually the grain direction is kept parallel to the binding edge so that when turning pages, there is less stiffness perpendicular to the binding edge. For Folders the grain isould be parallel to the folding direction in order to pervent cracking at the folds.
Paper gloss is one of the visual impressions you get when you look at a paper surface.
- paper appears glossy when most of the incident light is reflected directly.
- paper appears satin when most of the light is reflected diffusely.
Standardised gloss measurement of paper works according to this principle. The measuring device has a light source and a detector. A standard light source at the angle of incidence (e.g. 75%) emits light onto the test object. The amount of light reflected at the same angle of exit is measured. The amount of light recorded by the detector is expressed as a percentage of the amount of light emitted and provides a conclusive statement about the gloss of a substrate. Well-known measurement methods are Lehmann and Taber
Typical gloss values:
| Quality | Gloss at 75° |
|---|---|
| uncoated paper | 4-6 |
| smooth coated paper | 10-30 |
| gloss coated paper | 65-86 |
There are various methods for determining the smoothness or roughness of a surface. The best-known methods in europe are BEKK, BENDTSEN and PARKER Printsurf (PPS). These three methods measure the air flow between an aperture and the substrate surface.
- The BEKK (sec) method is mainly used to measure smooth and coated papers. The longer it takes for the standardised amount of air to escape between the paper and the aperture, the smoother the paper is.
- The BENDTSEN method is used for rough surfaces and uncoated papers. Here, air is pressed through a gap between the paper and a measuring ring. The unit of measurement is the amount of air (ml) that escapes within one minute. The higher the value, the rougher the paper.
- PARKER Printsurf (PPS) also involves forcing a stream of air between the substrate and a measuring ring. Here, the lower the measured value, the smoother the paper.
Comparative analyses of papers should always be carried out using one measurement method.
opacity
The opacity of paper describes how translucent or transparent a sheet of paper is – in other words, how much light can pass through a sheet. High opacity means that the paper is not very transparent and the print on the page is easy to read without the markings on the other side distracting the reader. Therefore, the opacity of a paper is particularly important when printing on both sides.
The opacity of paper is measured according to ISO standard 2471 and expressed as a percentage. With an opacity of 100 per cent, the paper is opaque/opaque. A sheet of paper with a light transmission of one per cent therefore has an opacity of 99 per cent.
woodfree coated (WFC)
The term ‘wood-free papers’ may seem a little bit confusing, as these papers are also made from cellulose fibres and are therefore based on wood, but they do not contain any wood pulp (lignin) and therefore hardly become yellow at all as they age.
‘Coated’ means that a special coating has been applied to create a smooth and even surface that is particularly well suited for printing high-quality images and text.
woodfree uncoated (WFU)
are woodfree papers (see also WFC) without binder application.
They are used for printing a wide range of products, such as advertising materials, books, maps and office papers. They come in a wide range of weights, from low grammages to cardboard-like paper.